Objective:To evaluate the applicable value of respiratory impedance for the stage 0 of chronic obstructive pulmonary disease (COPD). Methods:Respiratory impedance was measured by impulse oscillometry (IOS) in 41 cases at stage 0 of COPD (NHLBI/WHO, 2001 Standard) and their conventional pulmonary function values were compared with 42 healthy subjects. Both groups had no significant deviation in age, stature and avoirdupois, etc. Master-Screen pulmonary function test system (Jaeger Co, Germany) were used to determine IOS parameters including viscous resistance of 5Hz, 20 Hz and 35Hz (R5, R20, R35), reactance of 5Hz, 35 Hz (X5, X35 ), resonant frequency (Fres) ,total respiratory impedance (Zrs) and routine pulmonary function values including forced expiratory volume in one second to predicted value (FEV1%), forced expiratory volume in one second to forced vital capacity ratio( FEV1/FVC%), maximal mid-expiratory flow (MMEF%),V25% and V50%. Results:Both groups had no significant deviation in FEV1%, FEV1/FVC% and X35 (P>0.05). It was increased significantly in viscous resistance of 5Hz, 20 Hz and 35Hz (R5, R20, R35) in COPD group than that in healthy group (P<0.01). So were Fres and Zrs (P<0.01). MMEF%,V25%,V50% and reactance of 5Hz (X5) in COPD group were marked lower than that in healthy group (P<0.01). The sensitivity of MMEF%, V25%,V50% was higher than others, but its specificity was lower. In parameters of IOS, Fres was the most sensitive index for diagnosis of the small airway function and its specificity was higher than that of MMEF%, V25%,V50%. Conclusion:In the risk case at the stage 0 of COPD, MMEF%, V25% and V50% could be decreased, but Fres, R5,R5-20 could be increased in spite of FEV1% and FEV1/FVC% in normal range.